1. Field of the Invention
The present invention relates to a semiconductor device having a BPSG (boron-doped phosphorus silicate glass) interlayer insulating film, and particularly, to a method of minimizing the oxidization of a semiconductor substrate and gate electrode during a reflow process.
2. Description of the Related Art
When forming highly-integrated semiconductor devices, it is necessary to minimize the extension of diffusion layers by carrying out a heat treatment at a low temperature within a short time. The highly-integrated semiconductor devices employ fine design rules, and therefore, interlayer insulating films of high aspect ratios must be formed for them without voids. To achieve this, the interlayer insulating films are usually made of BPSG. A BPSG film is easy to carry out a reflow process thereon at a low temperature within a short time. The reflow process is usually carried out in a water vapor atmosphere to decrease the temperature and time of the reflow process. The water vapor atmosphere, however, oxidizes semiconductor regions and gate electrodes, or thickens and varies gate insulating films, or thins and varies the gate electrodes. In addition, hydrogen (H) in the water (H2O) vapor atmosphere diffuses into gate oxide films, to deteriorate the reliability of the gate oxide films.
FIG. 1A is a sectional view showing a semiconductor device before a reflow process. The semiconductor device consists of a silicon substrate 1 on which a gate electrode is formed. A BPSG film is formed on the gate electrode. More precisely, a silicon oxide film 2 serving as a gate insulating film is formed on the substrate 1. On the film 2, there is a lamination of a polysilicon film 3 and a tungsten silicide (WSi2) film 4 serving as the gate electrode. On the film 4, there is an oxide film 5 for protecting the gate electrode. The gate electrode is shaped by photolithography and by anisotropic etching such as reactive ion etching (RIE). The BPSG film 7 is formed over the film 5, an exposed area of the substrate 1, and the side walls of the films 2 to 4. Before forming the BPSG film 7, a silicon oxide film may be formed to prevent boron and phosphorus in the BPSG film 7 from diffusing into the substrate 1 and films 2 and 3. Such a silicon oxide film is incapable of preventing the oxidization of the substrate 1 and films 3 and 4. The top surface of the BPSG film 7 has irregularities corresponding to the height of the films 2 to 5. To flatten the top surface of the BPSG film 7, a reflow process is carried out. At this time, the substrate 1 and films 3 and 4 are oxidized as exaggeratedly shown in FIG. 1B. The substrate 1 is sometimes oxidized to 8 nm from an interface with the BPSG film 7, to form oxidized areas 21. The oxidized areas 21 extend under the edges of the film 2 to thicken the film 2 at the edges. The film 3 is oxidized on the side faces thereof that are in contact with the BPSG film 7 and at edges that are in contact with the film 2. The film 3 is more easily oxidized than the substrate 1, and therefore, the oxidized depth of the film 3 is deeper than that of the substrate 1. The conductivity of the film 3 is lost at the oxidized parts, to shorten an effective gate length. The oxidization at the edges of the film 3 thickens the edges of the film 2. The film 4 is oxidized on the side faces thereof that are in contact with the BPSG film 7. The film 4 is more easily oxidized than the substrate 1, and therefore, the oxidized depth of the film 4 is deeper than that of the substrate 1. The conductivity of the film 4 is lost at the oxidized parts 24, to increase the resistance of the gate electrode.
FIG. 2A is a sectional view showing a semiconductor device before a reflow process. The semiconductor device has a silicon substrate 1 and a double gate electrode. On the silicon substrate 1, a silicon oxide film 2 serving as a gate insulating film is formed. On the film 2, a polysilicon film 3 serving as a floating gate is formed. On the film 3, an oxide film 9 is formed. On the film 9, a lamination of a polysilicon film 10 and a tungsten silicide (WSi2) film 4 is formed to serve as a control gate. On the film 4, an oxide film 12 is formed. A BPSG film 7 is formed on the film 12, an exposed area of the substrate 1, and the side walls of the films 2, 3, 9, 10, and 4. To flatten the top surface of the BPSG film 7, a reflow process is carried out. As exaggeratedly shown in FIG. 2B, the reflow process oxidizes the substrate 1 and films 3, 10, and 4. The oxidized areas 21 of the substrate 1 extend under the edges of the film 2, to thicken the edges. The film 3 is oxidized on the side faces thereof that are in contact with the BPSG film 7 and at edges that are in contact with the films 2 and 9, to shorten the gate length of the floating gate. The oxidized edges of the film 3 thicken the edges of the films 2 and 9. The film 10 is oxidized on the side faces thereof that are in contact with the BPSG film 7 and at edges that are in contact with the film 9, to shorten the gate length of the control gate. The oxidized edges of the film 10 thicken the edges of the film 9. The film 4 is oxidized on the side faces thereof that are in contact with the BPSG film 7. The film 4 is more easily oxidized than the substrate 1, and therefore, the oxidized depth of the film 4 is deeper than that of the substrate 1. The conductivity of the film 4 is lost at the oxidized parts 24, to increase the resistance of the gate electrode.